It is necessary in video imaging systems to balance the output of the three color channels such that white input to the system will appear as white output. In video cameras, color or white balance correction is often accomplished by a white balance circuit that is activated while the video camera images a white card (or some other white reference) in order to obtain a white reference level. The white balance is adjusted in some conventional film-to-video image conversion systems automatically on power up when the imaging head is looking at the open film gate, i.e. before the film to be converted is introduced in the optical path of the film-to-video conversion system. In either case, the white balance adjustment is generally set at a prescribed illumination level and other illumination levels are left to fall where they may as far as white balance is concerned, i.e. no compensation in white balance is provided for changes in the illumination level.
To be required to reset the white balance whenever changes in illumination conditions occur in the above-described systems is clearly inconvenient and impractical. It is therefore an object of the invention to provide a system for automatically correcting the white balance in a video system to compensate for any change in illumination that may occur in the video system.